Polyfluorene Polymer Research Articles

Simultaneous Force and Fluorescence Spectroscopy on Single Chains of Polyfluorene: Effect of Intra-Chain Aggregate Coupling

Conjugated polymer chains in compact conformations or in films exhibit spectral features that can be attributed to interactions between individual conjugated segments of the chain, including formation of aggregates or excimers. Here, we use atomic force microscopy (AFM) on single chains of the conjugated polymer polyfluorene (PFO) to control the intersegment interactions by mechanically unfolding the chain. Simultaneously with the force spectroscopy we monitor fluorescence from the single PFO chains using a fluorescence microscope. We found that mechanical stretching of the chain causes disappearance of the green emission band. This observation provides evidence that the green emission originates from an intrachain aggregated state on the self-folded chain, which is decoupled by the stretching. In addition, the stretching upon laser irradiation leads to the appearance of additional features in the force spectra, small force peaks in the initial stages of the unfolding. These features are attributed to a combination of excitonic and van der Waals coupling of a ground-state intrachain aggregate.

Intense pH Sensitivity Modulation in Carbon Nanotube-Based Field-Effect Transistor by Non-Covalent Polyfluorene Functionalization.

We compare the pH sensing performance of non-functionalized carbon nanotubes (CNT) field-effect transistors (p-CNTFET) and CNTFET functionalized with a conjugated polyfluorene polymer (labeled FF-UR) bearing urea-based moieties (f-CNTFET). The devices are electrolyte-gated, PMMA-passivated, 5 µm-channel FETs with unsorted, inkjet-printed single-walled CNT. In phosphate (PBS) and borate (BBS) buffer solutions, the p-CNTFETs exhibit a p-type operation while f-CNTFETs exhibit p-type behavior in BBS and ambipolarity in PBS. The sensitivity to pH is evaluated by measuring the drain current at a gate and drain voltage of -0.8 V. In PBS, p-CNTFETs show a linear, reversible pH response between pH 3 and pH 9 with a sensitivity of 26 ± 2.2%/pH unit; while f-CNTFETs have a much stronger, reversible pH response (373%/pH unit), but only over the range of pH 7 to pH 9. In BBS, both p-CNTFET and f-CNTFET show a linear pH response between pH 5 and 9, with sensitivities of 56%/pH and 96%/pH, respectively. Analysis of the I-V curves as a function of pH suggests that the increased pH sensitivity of f-CNTFET is consistent with interactions of FF-UR with phosphate ions in PBS and boric acid in BBS, with the ratio and charge of the complexed species depending on pH. The complexation affects the efficiency of electrolyte gating and the surface charge around the CNT, both of which modify the I-V response of the CNTFET, leading to the observed current sensitivity as a function of pH. The performances of p-CNTFET in PBS are comparable to the best results in the literature, while the performances of the f-CNTFET far exceed the current state-of-the-art by a factor of four in BBS and more than 10 over a limited range of pH in BBS. This is the first time that a functionalization other than carboxylate moieties has significantly improved the state-of-the-art of pH sensing with CNTFET or CNT chemistors. On the other hand, this study also highlights the challenge of transferring this performance to a real water matrix, where many different species may compete for interactions with FF-UR.

Novel Amphiphilic Polyfluorene-Graft-(Polymethacrylic Acid) Brushes: Synthesis, Conformation, and Self-Assembly.

Novel polyfluorene polymer brushes with polymethacrylic acid side chains were obtained by atom transfer radical polymerization (ATRP) and activator generated by electron transfer (AGET) ATRP of tert-butyl methacrylate on polyfluorene multifunctional macroinitiator, followed by protonolysis of the tert-butyl groups of the side chains. Kinetics of polymerization and molecular weights were fully characterized. These polymer brushes luminesce in the blue region of the spectrum with high quantum yields (0.64–0.77). It was shown that the luminescence intensity of polymer brushes is higher than the luminescence intensity of the macroinitiator (0.61). Moreover, due to their amphiphilic nature, they can form unimolecular micelles when an alcohol solution of the polymer brush is injected into water. These properties can potentially be used in drug delivery and bioimaging.

Enhancing Carrier Mobility in Organic Light Emitting Diode Based Polyfluorene by Self-Assembled Monolayers Technique on Indium Tin Oxide

An organic compound (1'-4-iodophenyl) trihidroxysilane (OH-IPhSi) functionalized as hole transporting material was synthesized and used as interface self-assembled monolayers (SAM) on ITO film. To analyze the effect of a SAM layers on the characteristics an organic light emitting diode (Organic LED), a typical Organic LED device (ITO/OH-IPhSi/polymer/LiF/Al) was fabricated. The SAM-treatment ITO device performed a higher mobility than the bare ITO device. The carrier mobility properties were calculated by space-charge-limited current measurements (SCLC) technique for polyfluorene (PFO) polymer Organic LED devices.

Light-Emitting Field-Effect Transistors Based on Composite Films of Polyfluorene and CsPbBr3 Nanocrystals

Light-emitting organic field-effect transistors (LE-FETs) on the basis of composite films that consist of perovskite nanocrystals (CsPbBr3) embedded in a matrix of conjugated polymer—polyfluorene (PFO)—have been obtained, and their electrical and optical properties have been investigated. Output and transfer current-voltage characteristics (I-Vs) of FETs based on PFO : CsPbBr3 films (component ratio 1 : 1) have a slight hysteresis at temperatures of 100–300 K and are characteristic of hole transport. The hole mobility is ∼3.3 and ∼1.9 cm2/(V s) at the modes of the saturation and low fields, respectively, at 250 K and reaches ∼5 cm2/(V s) at 100 K. It has been shown that the application of pulsed voltage to LE-FETs based on PFO : CsPbBr3 can reduce the ionic conductivity and provide electroluminescence in this structure at 300 K.

Светоизлучающие полевые транзисторы на основе композитных пленок полифлуорена и нанокристаллов CsPbBr-=SUB=-3-=/SUB=-

Abstract—Light-emitting organic field-effect transistors (LE-FETs) on the basis of composite films that consist of perovskite nanocrystals (CsPbBr_3) embedded in a matrix of conjugated polymer—polyfluorene (PFO)—have been obtained, and their electrical and optical properties have been investigated. Output and transfer current-voltage characteristics (I-Vs) of FETs based on PFO : CsPbBr_3 films (component ratio 1 : 1) have a slight hysteresis at temperatures of 100–300 K and are characteristic of hole transport. The hole mobility is ∼3.3 and ∼1.9 cm^2/(V s) at the modes of the saturation and low fields, respectively, at 250 K and reaches ∼5 cm^2/(V s) at 100 K. It has been shown that the application of pulsed voltage to LE-FETs based on PFO : CsPbBr_3 can reduce the ionic conductivity and provide electroluminescence in this structure at 300 K.

Electron Transport with Mobility, μ > 86 cm2/(V s), in a 74 nm Long Polyfluorene.

The mobility of charges on conjugated polymers is a fundamentally important feature of these materials, but most fall far short of transport that might lead one to call them "molecular wires". A commonly identified bottleneck is flexible dihedral angles between repeat units. Here we find a very high mobility, μ > 86 cm2/(V s), for electrons attached to polyfluorene polymers in isooctane, despite the presence of varied dihedral angles. The present data suggest that interactions with the surrounding medium may be a principal determinant of charge mobility.

Diameter-Dependent Optical Absorption and Excitation Energy Transfer from Encapsulated Dye Molecules toward Single-Walled Carbon Nanotubes.

The hollow cores and well-defined diameters of single-walled carbon nanotubes (SWCNTs) allow for creation of one-dimensional hybrid structures by encapsulation of various molecules. Absorption and near-infrared photoluminescence-excitation (PLE) spectroscopy reveal that the absorption spectrum of encapsulated 1,3-bis[4-(dimethylamino)phenyl]-squaraine dye molecules inside SWCNTs is modulated by the SWCNT diameter, as observed through excitation energy transfer (EET) from the encapsulated molecules to the SWCNTs, implying a strongly diameter-dependent stacking of the molecules inside the SWCNTs. Transient absorption spectroscopy, simultaneously probing the encapsulated dyes and the host SWCNTs, demonstrates this EET, which can be used as a route to diameter-dependent photosensitization, to be fast (sub-picosecond). A wide series of SWCNT samples is systematically characterized by absorption, PLE, and resonant Raman scattering (RRS), also identifying the critical diameter for squaraine filling. In addition, we find that SWCNT filling does not limit the selectivity of subsequent separation protocols (including polyfluorene polymers for isolating only semiconducting SWCNTs and aqueous two-phase separation for enrichment of specific SWCNT chiralities). The design of these functional hybrid systems, with tunable dye absorption, fast and efficient EET, and the ability to remove all metallic SWCNTs by subsequent separation, demonstrates potential for implementation in photoconversion devices.

High-efficiency organic solar cells based on a halide salt and polyfluorene polymer with a high alignment-level of the cathode selective contact

The combination of halide salts with polyfluorene used as the buffer cathode in organic solar cells greatly improves the photovoltaic performance with the best power conversion efficiency being 11%.

Low-Temperature Single Carbon Nanotube Spectroscopy of sp3 Quantum Defects.

Aiming to unravel the relationship between chemical configuration and electronic structure of sp3 defects of aryl-functionalized (6,5) single-walled carbon nanotubes (SWCNTs), we perform low-temperature single nanotube photoluminescence (PL) spectroscopy studies and correlate our observations with quantum chemistry simulations. We observe sharp emission peaks from individual defect sites that are spread over an extremely broad, 1000-1350 nm, spectral range. Our simulations allow us to attribute this spectral diversity to the occurrence of six chemically and energetically distinct defect states resulting from topological variation in the chemical binding configuration of the monovalent aryl groups. Both PL emission efficiency and spectral line width of the defect states are strongly influenced by the local dielectric environment. Wrapping the SWCNT with a polyfluorene polymer provides the best isolation from the environment and yields the brightest emission with near-resolution limited spectral line width of 270 μeV, as well as spectrally resolved emission wings associated with localized acoustic phonons. Pump-dependent studies further revealed that the defect states are capable of emitting single, sharp, isolated PL peaks over 3 orders of magnitude increase in pump power, a key characteristic of two-level systems and an important prerequisite for single-photon emission with high purity. These findings point to the tremendous potential of sp3 defects in development of room temperature quantum light sources capable of operating at telecommunication wavelengths as the emission of the defect states can readily be extended to this range via use of larger diameter SWCNTs.

Diameter-Dependent Excitation Energy Transfer for Enhanced Semiconducting Single-Walled Carbon Nanotube Solar Photoconversion

Tremendous progress and innovation has ushered the incorporation of semiconducting single-walled carbon nanotubes (s-SWCNTs) as active components into organic/inorganic/hybrid solar cells and thermoelectrics. For solar photoconversion, strong and tunable absorption is necessitated, suggesting that s-SWCNTs can play a role as light-harvesting components, however the narrow and distinct excitonic optical transitions found in s-SWCNT systems limit broad-band absorption. As such, we introduce encapsulation of small molecules within the endohedral volume of s-SWCNTs as a route to increase and extend light absorption in regions outside s-SWCNT excitonic transitions. Creation of this self-assembled supra-molecular system can strongly modify the opto-electronic properties of both the s-SWCNT and encapsulated molecule. We investigate excitation energy transfer occurring from encapsulated dye molecules to s-SWCNTs, which results in luminescent SWCNT excitons. We exploit the high-throughput and chiral selectivity of polyfluorene wrapping methods to generate dispersions of s-SWCNTs with dye encapsulation. We demonstrate that the selectivity of the polyfluorene polymer for semiconducting chiral distributions is not affected by the presence of encapsulated molecules. Tracking the excited state transient absorption signatures of excitation energy transfer provides detailed information regarding dynamics of both the encapsulated dye and s-SWCNTs. We observe sub-picosecond excitation energy transfer of excitons generated in dye molecules to the surrounding s-SWCNTs. Photoluminescence excitation maps reveal that the absorption of encapsulated dye molecules depends sensitively on the diameter of the s-SWCNT in which they are encapsulated. We consider a simple molecular exciton model to describe aggregate formation within the s-SWCNT endohedral volume, which reveals that the dye molecules adopt unique aggregate structures that are defined both by a competition between intermolecular interactions between the dye molecules and confinement effects due to the s-SWCNT diameter. Small molecule encapsulation in s-SWCNTs serves as a strong platform to control intermolecular interactions in small molecule systems through confinement, which opens up the possibility of enhancing photo-induced extraction of energy and charge in s-SWCNTs solar photoconversion systems.

Modification of the terahertz electromagnetic response of the semiconducting polymer polyfluorene by graphene oxide particles

The terahertz electromagnetic response of composite films based on the semiconducting polymer polyfluorene and graphene oxide particles is examined by terahertz time-domain spectroscopy. The spectra of complex permittivity in the range of 0.3–2.8 THz are measured for composite polyfluorene films with 7, 26, 50.6, and 73 wt % graphene oxide. It is found that the terahertz absorption coefficient and the real part of conductivity of the polyfluorene–graphene oxide composite increases significantly with the graphene oxide particles content increase.

Fabrication of “Clickable” Polyfluorene Nanowires with High Aspect Ratio as Biological Sensing Platforms

“Clickable” nanowires with well-defined and uniform structures made of conjugated polyfluorene polymers were successfully fabricated by single particle nanofabrication technique (SPNT). Poly[(9,9-dihex-5-yn-1-ylfluorenyl-2,7-diyl)-co-(9,9′-di-n-octylfluorenyl-2,7-diyl)] (F6E8) and poly[(9,9-dihex-5-yn-1-ylfluorenyl-2,7-diyl)-co-(2,2′-bithiophene)] (F6E2T) underwent an efficient cross-linking reaction upon irradiation, resulting in formation of one-dimensional nanostructures with high and desired aspect ratio reaching up to 200. Alkyne groups on the surface of nanowires were functionalized effectively by click reaction with fluorescent 5-TAMRA-PEG3-azide, which was confirmed by confocal microscopy. Substrates functionalized with the nanowires provide dramatic expansion of “clickable” surface area immobilized directly with TAMRA, and the fluorescence resonance energy transfer (FRET) processes between TAMRA and nanowire backbones are demonstrated as biological sensing platforms.

(Invited) Pristine Electronics Grade Semiconducting Carbon Nanotubes By Switching the Rigidity of the Wrapping Polymer Backbone

In order to widely use single-walled carbon nanotubes (SWCNTs) as the semiconducting material in electronic devices, it is essential to separate the desirable semiconducting SWCNTs from as-synthesized electronically heterogeneous mixtures of metallic (m-) and semiconducting (s-) SWCNTs. Among the sorting methods developed, polyfluorene polymers have been studied as semiconducting-selective agents with selectivity for chirality, diameter and electronic type. Developing an understanding for the factors that lead to strong selective interactions between the conjugated polymer and the semiconducting tubes is evolving but far from complete. We choose a commonly used commercially available polyfluorene derivative PFO-BPy as a model polymer to directly probe the effect of chain rigidity on the wrapping/unwrapping process on the s-SWCNTs. Our choice of this model system is based on its commercial availability, outstanding sorting selectivity, availability of functional groups in the backbone to alter rigidity, and recent device results. Recently, we demonstrated the extraordinarily electronic-type selectively of the polyfluorene derivative PFO-BPy by measuring the on/off ratio of FETs, where zero metallic nanotubes were encountered in the measurement of more than 5,000 s-SWCNTs.1-3 Using the similar polymer poly(9,9-dioctylfluorene-2,7-diyl) (PFO), Bindl et al., demonstrated that even after several aggressive rinsing steps to remove free or excess polymer using ultracentrifugation a significant amount of PFO remained bound to the SWCNT. 4 These remaining polymer residues are expected to increase the contact resistance at the metal-nanotube interface of SWCNT FETs, ultimately limiting the conductance of these FETs at sub-100 nm channel lengths. A more complete removal of adsorbed polymer residues from the SWCNT surface is one possible approach towards improving contact resistance and reducing variation in device performance.5 Recently we demonstrated6 an effective yet simple and mild method to remove post-sorting, and the wrapping PFO-BPy copolymer from the surface of s-SWCNTs by switching the backbone rigidity. We used chelation chemistry to complex pentacarbonylrhenium chloride (Re (CO)5Cl) to the bipyridine (BPy) moiety in the wrapping copolymer backbone. We showed the effectiveness of the chelation chemistry on both large and small diameter tubes. The chain-stiffness of the PFO-BPy changes upon complexation, likely providing the driving force to overcome the π-π and electronic interactions7 which adhere the polymer to the nanotube. Optical absorbance, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and large-scale molecular dynamics simulations were used to characterize the extent of PFO-BPy removal and configurational changes to the PFO-BPy after Re(CO)5Cl complexation (PFO-BPy:Re).The ability to switch the rigidity of the PFO-BPy in order to unwrap from the nanotubes opens up the door to new design strategies where specific functional groups can be placed on the polymer backbone to chemically alter the rigidity. Here, we present recent results on direct experimental measurements of changes in stiffness of the polymer backbone and its aggregation state upon metal complexation. 1. Brady, G. J.; Joo, Y.; Roy, S. S.; Gopalan, P.; Arnold, M. S. Appl. Phys. Lett. 2014, 104, (8), 083107. 2. Joo, Y.; Brady, G. J.; Arnold, M. S.; Gopalan, P. Langmuir 2014, 30, (12), 3460-3466. 3. Mistry, K. S.; Larsen, B. A.; Blackburn, J. L. ACS nano 2013, 7, (3), 2231-2239. 4. Bindl, D. J.; Shea, M. J.; Arnold, M. S. Chem. Phys. 2013, 413, 29-34. 5. Wang, W. Z.; Li, W. F.; Pan, X. Y.; Li, C. M.; Li, L. J.; Mu, Y. G.; Rogers, J. A.; Chan-Park, M. B. Adv. Funct. Mater. 2011, 21, (9), 1643-1651. 6. Joo, Y.; Brady, G. J.; Shea, M. J.; Oviedo, M. B.; Kanimozhi, C.; Schmitt, S. K.; Wong, B. M.; Arnold, M. S.; Gopalan, P. ACS Nano 2015, 9, (10), 10203-10213. 7. Nish, A.; Hwang, J. Y.; Doig, J.; Nicholas, R. J. Nature nanotechnology 2007, 2, (10), 640-6.

Carbon Nanotube Array Field Effect Transistors with High Current Density and On/Off Ratio

Single walled carbon nanotubes (SWCNTs) exhibit extraordinarily high current carrying capacity and suitable band gaps for logic and thin film field-effect transistors (FETs). Progress in honing the exceptional properties of SWCNTs at the multi-tube level, however, has been dampened by incremental advances in the materials science of electronic type sorting and assembly of SWCNTs. To address these challenges we leveraged the exceptional semiconducting sorting fidelity of polyfluorene polymers and we pioneered an alignment technique known as Floating Evaporative Self-Assembly (FESA).1 Recently, we demonstrated individually placed and uniformly pitched purely semiconducting SWCNT arrays using these techniques, allowing for negligible current lost to scaling when the arrays were implemented as the channel in FETs. The SWCNT array FETs exhibited the highest on-state conductance of 261 μS/μm with a simultaneous on/off conductance ratio exceeding 105 for a multi-tube SWCNT FET.2 Here we present our recent optimization of the contact resistance at the palladium-SWCNT interface, which enables a substantial increase in device on-current density compared to our previous report. For SWCNTs, the quality of the contact-semiconductor interface is highly dependent on the SWCNT packing density, film cleanliness, and surface energy, where any non-idealities may lead to poor palladium film adhesion, or even complete film delamination.3 To address the contact challenge we perform surface treatments on the SWCNT array prior to depositing palladium contacts, which involves rinsing the SWCNT arrays in specific solvents and annealing the films in vacuum. In XPS and optical absorbance spectra, we observe significant mass reduction of the polyfluorene wrapper while maintaining the quality and alignment of the SWCNT array. AFM height maps of the palladium contacts deposited on top of the surface treated SWCNTs demonstrate nearly ideal conformation of the palladium on the SWCNT array. The surface-treated SWCNT array FETs exhibit more than 10-fold improvement in the device on-conductance compared to non-treated samples. [1] Y. Joo, G. J. Brady, M. S. Arnold, and P. Gopalan, Langmuir 30 (12), 3460 (2014). [2] G. J. Brady, Y. Joo, M. Y. Wu, M. J. Shea, P. Gopalan, and M. S. Arnold, ACS Nano 8 (11), 11614 (2014). [3] V. Perebeinos and J. Tersoff, Physical Review Letters 114 (8), 4 (2015).

(Invited) Insight into Carbon Nanotube Surface Structures for PV Applications

Carbon nanotube (CNT) surface chemistry and surface structures impact their potential roles in photovoltaics in a number of ways. These include defining chirality-specific segregation behaviors that enable isolation of the required pure chiralities. Surface structures also define film morphologies and ordering that ultimately determine how intertube interactions facilitate or inhibit charge and energy flow. We present photoluminescence imaging results as a direct probe of these relevant surface structures. A demonstration of phase-dependent segregation in surfactant compositions at the nanotube surface will be shown as a determining factor in the mechanism of new aqueous two-phase separations approaches. We will also show how variations in polyfluorene polymer structures impact their surface morphologies, ultimately determining the ability to transport excitation energy and charge within thin-film nanotube-based PV devices.

Circular dichroism, surface-enhanced Raman scattering, and spectroscopic ellipsometry studies of chiral polyfluorene-phenylene films

To understand the chiroptical responses of polyfluorene–phenylene (PFP) films, circular dichroism (CD), surface-enhanced Raman scattering (SERS), attenuated total reflection (ATR), and spectroscopic ellipsometry studies were performed. The pristine PFP films exhibited significant CD at room temperature, and the chirality increased by thermal annealing at 120 °C (above the glass transition temperature). SERS and ATR spectra indicated that the chirality enhancement was due to the rearrangement of the polymer backbones on the glass substrate by thermal annealing and/or self-organization. In addition, comparison of PFP and polyfluorene polymers indicated that molecular structure of polymer backbone plays an important role in the chiroptical responses of the chiral polymers.

A Streak Camera Study of Amplified Spontaneous Emission in Polyfluorene Thin Film

ABSTRACT : We report on the photoluminescence (PL) properties of poly[2,7-(9-9-dioctylfluorene)] (PF) thin film under strong optical pumping using a streak camera system. When the excitation energy density increases -above 72 μJ·cm 2 , the emission spectrum becomes narrower and PL decay curve comes to be faster simultaneously. These behaviors are clear evidence of Amplified Spontaneous Emission (ASE) due to a waveguided Stimulated Emission in slab structure of thin film. ASE threshold of 72 μJ·cm -2 is comparable with previous reports and PF is attractive as a gain medium for plastic lasers. Since the first investigation on the electroluminescence (EL) of poly(p-phenylenevinylene), 1 semiconducting π-conjugated polymers have become more attractive materials for applications in optoelectronic devices such as light emitting diodes (LED), plastic lasers, and optical amplifiers. 2-5 Among the π-conjugated polymers, particularly a number of polyfluorene polymers and its copolymers could be promising candidates for realizing these devices due to the unique properties of good thermal and oxidative stability, good solubility in common organic solvents, excellent charge transport ability, and high photoluminescence (PL) quantum efficiency.

Detection of DNA hybridization using a cationic polyfluorene polymer as an enhancer of luminol chemiluminescence

The water-soluble conjugated polymer poly[(9,9-di(3,3-N,N-trimethylammonium)propylfluorenyl-2,7-diyl)-alt-co-(1,4-phenylene)] dibromide (referred to as PFP+) is shown to represent a useful reagent for the detection of DNA hybridization. The intensity of the chemiluminescence (CL) of the luminol-H2O2 system is strongly enhanced in the presence of PFP+, but this enhancement is inhibited after electrostatic interaction between PFP+ and DNA as shown for oligomers of lengths between 12 and 35 bases. CL decreases more strongly in the presence of ds-DNA than in the presence of ss-DNA. Hybridization between probe DNA and target DNA results in a weaker effect of PFP+ and in reduced CL emission. These finding are exploited in a homogeneous CL based assay for medium-length DNA (12 to 35 bp). The detection limit (at a signal-to-noise ratio of 3σ) is 3.0 pM.

Polymer-Decorated Carbon Nanotubes as Transducers for Label-Free Photonic Biosensors.

A biosensor taking advantage of the optical properties of sorted carbon nanotubes has been developed. A polyfluorene polymer bearing azido groups was synthesized and used for the selective extraction of semi-conducting nanotubes from the bulk population. The resulting polymer-decorated nanotubes were then conjugated by click-chemistry to a ligand unit (biotin), and the sensing properties of the biotinylated nanotubes were investigated by photoluminescence measurements, upon interaction with the streptavidin target.